Polyaspartic acid has shown utility in a wide variety of applications such as scale inhibition in water treatment, tartar control in oral hygiene products, as a detergent builder and anti-redeposition agent in detergent formulations, as a corrosion inhibitor, and in agricultural applications.
Most of the current methods of producing polyaspartic acid or salts of polyaspartic acid produce polymers of relatively low molecular weight. U.S. Pat. No. 5,221,733 to Koskan teaches the thermal polymerization of aspartic acid to polysuccinimide and its subsequent hydrolysis with aqueous base to afford polyaspartates of molecular weight of about 3000 to about 6000 daltons. In U.S. Pat. No. 5,288,783, Wood describes the thermal polymerization of ammonium salts of maleic and fumaric acid to produce relatively low molecular weight polyasparatate of about 2000 daltons after base hydrolysis.
Some attempts to obtain higher molecular weight copolymers of maleic acid, ammonia and diamines by thermal polymerization are described in U.S. Patent No. 5,286,810 also to Wood. However, the Wood method is complex, requiring multiple steps in manipulating the reaction mixture during the polymerization and the polymers produced are highly colored and of modestly higher molecular weights than are obtained in the absence of diamine.
In many applications, performance characteristics are a function of the molecular weight of a polymer. It is often desirable to be able to produce a range of molecular weights to optimize the performance of a particular polymer to the application in which it will be used. Higher molecular weight polymers often have greatly different performance characteristics than the corresponding lower molecular weight polymers. In some applications, performance characteristics are adversely affected at higher molecular weights, whereas in other applications, performance characteristics may be enhanced.
The chemical modification of polysuccinimide to produce useful materials is well known. Koskan, et al., in U.S. Pat. Nos. 5,057,597, 5,116,513, 5,219,952and 5,221,733 describe inexpensive methods for manufacturing polysuccinimide and polyaspartic acid. In the article by Neri, et al., "Synthesis of alpha, beta-Poly [(2-hydroxyethyl) DL-aspartamide], a New Plasma Expander," Journal of Medicinal Chemistry, Vol. 16, pp 893-897 (1973), the authors describe the modification of polysuccinimide with ethanolamine to produce yet another useful product.
Fujimoto et al. in U.S. Pat. No. 3,846,380 describe the formation of modified polypeptides having hydrophobic and hydrophilic substituents as side chains by reacting polysuccinimide with at least one primary or secondary aliphatic amine and then hydrolyzing the resulting polyamide derivative with alkali to produce polypeptides that are useful as surface active agents.
However, there still exists a need for a simplified and cost-effective method of producing higher molecular weight soluble crosslinked polyaspartates. The present method satisfies that need.